U.S. Pat. Nos. 3,409,579 and 3,676,392 disclose binder compositions made available as two-package systems comprising a resin component in one package and a hardener component in the other package. The entire contents of each of these two U.S. patents are incorporated herein by reference. The resin component comprises an organic solvent solution of phenolic resin. The hardener component comprises a liquid polyisocyanate having at least two isocyanate groups per molecule. At the time of use, the contents of the two packages may be combined first and then mixed with the sand aggregate, or preferably the packages are sequentially admixed with sand aggregate. After a uniform distribution of the binder on the sand particles has been obtained, the resulting foundry mix is molded into the desired shape.
In U.S. Pat. No. 3,409,579, the molded shape is cured by passing a gaseous tertiary amine through it. In U.S. Pat. No. 3,676,392, curing is effected by means of a base having a pKb value in the range of about 7 to about 11 as determined by a method described by D. D. Perrin in Dissociation Constants of Organic Bases in Aqueous Solution (Butterworths, London 1965). The base is introduced originally into the resin component before it is mixed with hardener, or it may be introduced as the third component of a three-package binder system comprising in separate packages the resin component, the hardener, and the base.
In both U.S. Pat. Nos. 3,409,579 and 3,676,392, the preferred phenolic resins contain benzylic ether resins along with other reaction products. Benzylic ether resins are condensation products of a phenol having the general formula: ##STR1## wherein A, B, and C are hydrogen, hydrocarbon radicals, oxyhydrocarbon radicals, or halogen, with an aldehyde having the general formula R'CHO wherein R' is a hydrogen or a hydrocarbon radical of 1 to 8 carbon atoms, prepared in the liquid phase at temperatures below about 130.degree. C. in the presence of catalytic concentrations of a metal ion dissolved in the reaction medium. The preparation and characterization of these resins is disclosed in greater detail in U.S. Pat. No. 3,485,797, the entire contents of which is incorporated herein by reference. The phenolic resin component of the binder composition is, as indicated above, generally employed as a solution in an organic solvent.
The second component or package of the binder composition comprises an aliphatic, cycloaliphatic, or aromatic polyisocyanate having preferably from 2 to 5 isocyanate groups. If desired, mixtures of polyisocyanates can be employed. Less preferably, isocyanate prepolymers formed by reacting excess polyisocyanate with a polyhydric alcohol, e.g., a prepolymer of toluene diisocyanate and ethylene glycol, can be employed. Suitable polyisocyanates include the aliphatic polyisocyanates such as hexamethylene diisocyanate, alicyclic polyisocyanates such as 4,4'-dicyclohexylmethane diisocyanate, and aromatic polyisocyanates such as 2,4- and 2,6-toluene diisocyanate, diphenylmethane diisocyanate, and dimethyl derivatives thereof. Further examples of suitable polyisocyanates are 1,5-naphthalene diisocyanate, triphenylmethane triisocyanate, xylylene diisocyanate, and the methyl derivatives thereof, polymethylenepolyphenyl isocyanates, chlorophenylene-2,4-diisocyanate, and the like. Although all polyisocyanates react with the phenolic resin to form a cross-linked polymer structure, the preferred polyisocyanates are aromatic polyisocyanates and particularly diphenylmethane diisocyanate, triphenylmethane triisocyanate, and mixtures thereof.
The polyisocyanate is employed in sufficient concentrations to cause the curing of the phenolic resin. In general, the polyisocyanate will be employed in a range of 10 to 500 weight percent of polyisocyanate based on the weight of the phenolic resin. Preferably, from 20 to 300 weight percent of polyisocyanate on the same basis is employed. The polyisocyanate is employed in liquid form. Liquid polyisocyanates can be employed in undiluted form. Solid or viscous polyisocyanates are employed in the form of organic solvent solutions, the solvent being present in a range of up to 80% by weight of the solution.
The bench life of an aggregate binder can be defined as the maximum permissible time delay between mixing the binder components together with an aggregate such as sand and the production of acceptable products therefrom by curing. In order to extend the bench life of the above binder systems before they are contacted with the catalytic component, various materials have been suggested. Phthaloyl chloride is currently being commercially employed for such purpose but is not entirely satisfactory because of life shortening effects of moisture and/or elevated temperatures and/or impurities on the sand. Although great improvements in bench life have been obtained through the use of phosphorus halides as described in copending application Ser. No. 575,208, now U.S. Pat. No. 4,540,724, such halides are relatively expensive and/or may produce an unpleasant odor.